The generation of electrical signals representing graphic data has been a subject of investigation and study for many years. Applications of developments in the field of electrographics are quite numerous and promising. For example, graphic data in digital form may be treated by computer in providing graphic design problem analysis. Similarly, digitalized graphic information may be stored in computer memory or transmitted between remote stations via telecommunication links.
The generation of electrographic signals is initiated at a man-machine interface which generally is present as a surface upon which graphic data is manually developed. For the most part, such development occurs in the same fashion as graphics are generated utilizing paper, a stylus representing a writing instrument being drawn across the surface to form informational characters or designs. The surfaces upon which this drawing takes place are commonly known as "digitizers". The digitizers respond to the coordinate position of the stylus held by the operator and generate analog coordinate signals which are appropriately treated and converted to digital form for transmission.
For the most part, digitizers have been fashioned as composite structures wherein a grid formed of two spaced arrays of mutually orthogonally disposed fine wires is embedded in an insulative carrier. One surface of this structure serves to receive a stylus input which is converted to coordinate signals. Various methods have been devised for generating coordinate defining signals as a stylus-grid interaction, for example, a magnetostrictive effect may be established between stylus and grid or a capacitive coupling effect may be evoked between these components.
The use of such grid structures, while providing accurate, linear output coordinate signals necessarily involve intricate structures which are expensive to fabricate and prone to damage in the normal course of use. Further, for many applications it is desirable that the digitizer be fabricated as a highly transparent composite sheet. The grid structures within the composite structures, however, militate against achieving such desired transparency.
Another principal approach to the design of digitizers looks to the use of resistive surface coatings. An immediately apparent advantage of this approach resides in the inherent simplicity of merely providing a resistive surface upon a supportive substrate such as glass or plastic. Further, the resistive coating may be transparent to permit an expanded range of industrial applications.
Unfortunately, designers have encountered a variety of technical problems in adopting the resistive layer to provide cordinate output signals. Among these problems has been the non-linear nature of these coordinate read-outs. A precise one-to-one correspondence is required between actual stylus position and the resultant coordinate signals. However, a pin cushion form of distortion, among others, has been encountered by investigators causing the achievement of linearity of output to become an elusive goal. Various forms of correction have been developed; however, each such correction has been at the expense of losing a desired operational attribute or feature of the digitizer. Among these features desired for the digitizer product is a capability of "writing" with the stylus not touching the surface of the digitizer. Additionally, as indicated above, it is desirable that the digitizer be fabricable as a highly transparent surface. Further, it is most desirable that the digitizer work in conjunction with a sheet of opaque paper such that the operator may draw or make positional visual inputs upon the sheet of paper while, simultaneously, the digitizer provides real time coordinate output signals. Next, the structure of the digitizer must remain simple and immune from the wear and related vagaries encountered in common drafting utilization. In the latter regard, where composite structures requiring separation of resistive surfaces followed by flexure of one into the other are evolved, not only the cost of the digitizer becomes elevated but also the operational life and general reliability thereof become compromised.
Another desirable design aspect for digitizers resides in the development of signal conditioning circuitry accommodating typically encountered common mode noise. Where resistive surfaces are employed, various forms of ambient noise or spurious signals will be developed which will be manifested as error unless corrected. Of course, such accommodation necessarily is at the expense of more elaborate and costly circuit design. In the same regard, a minimization of the number of circuit components required to achieve accurate coordinate identification permits enhanced marketing opportunities for the digitizer products.
In addition to carrying out "writing" functions through the use of an operator held stylus or the like in conjunction with a digitizer surface, it very often is desirable for the operator to identify precise coordinate points upon a plan or drawing used in conjunction with a transparent digitizer surface. The stylus conventionally utilized with the digitizers may be somewhat inaccurate for this purpose inasmuch as the point thereof will have a finite thickness which tends to block the operator's vision from achieving a proper centering of the stylus over the desired point upon which coordinates are to be established. Accordingly, a desirable feature for such digitizers is some form of cursor which does not block the operator's view in establishing the coordinate position of a given point. In a copending application for U.S. patent Ser. No. 395,261 filed July 6, 1982, now U.S. Pat. No. 4,456,787 by the inventors hereof entitled "Electrographic System and Method", a digitizer structure is described which advantageously utilizes a transparent digitizer surface in conjunction with a stylus which is excited from an a.c. source to provide a field interaction with the digitizer surface. Through the utilization of coordinate identified edge contact switching in conjunction with the stylus and through the utilization of sum and difference output signal ratio treatment, the digitizer described therein is capable of achieving desirable linear output and of operating satisfactorily with stylus positioning at locations spaced from the digitizer surface itself. Further, the particular ratio form of signal treatment described therein serves to alleviate difficulties otherwise associated with common noise perturbations typically generated in the operation of such system. While representing an effective approach to the development of a practical digitizer structure, the signal treatment approach shown therein is one involving a considerable number of circuit elements a significant portion of which require accommodation for drift, offset phenomena and the like.